1. Field of the Invention
This invention relates to an apparatus for detecting misfire in an internal combustion engine on the basis of detection of an ion current through an ignition plug provided in a combustion chamber of the internal combustion engine.
2. Description of the Related Art
In internal combustion engines, a mixture of fuel and air is compressed in a combustion chamber and a spark is caused by applying a high voltage to an ignition plug provided in the combustion chamber to ignite and burn the mixture. Failure to cause burning of the mixture is called misfire. If misfire occurs, the desired power of the internal combustion engine cannot be obtained and the mixture containing a large amount of fuel flows into the exhaust system to corrode the exhaust pipe and other parts. Therefore, there is a need to detect a misfiring state and to warn a driver.
As a means for detecting misfire in an internal combustion engine, a circuit for detecting misfire by detecting an ion current flowing through an ignition plug provided in a combustion chamber is known. As fuel burns in the combustion chamber, molecules in the combustion chamber are ionized. When a voltage is applied to the ionized gas in the combustion chamber through the ignition plug, a small current flows, which is called ion current. The ion current is reduced to a very small value when misfire occurs. Occurrence of misfire can be determined by detecting such a change in ion current.
FIG. 8 is a diagram of this kind of conventional misfire detecting apparatus for use with an internal combustion engine.
Referring to FIG. 8, an ignition coil 1 has a primary coil 1a and a secondary coil 1b, and an ignition plug 2 provided in an internal combustion engine 2A is connected to a minus terminal of the secondary coil 1b. A plus terminal of the primary coil 1a is connected to a power source 4 while a minus terminal of the primary coil 1a is connected to the collector of a current switching transistor 3. The emitter of the transistor 3 is grounded, and the base of the transistor 3 is connected to a controller (not shown) for controlling combustion.
A misfire detection circuit 5 has a biasing capacitor 6 connected to a plus terminal of the secondary coil 1b to bias the ignition plug 2, a Zener diode 7 connected between the plus terminal of the secondary coil 1b and ground to set a voltage at which the capacitor 6 is charged, a charging diode 8 connected between the low potential side of the capacitor 6 and ground with its anode connected to the capacitor 6, an ion current converting resistor 9 also connected between the low potential side of the capacitor 6 and ground, and a capacitor 10 having one end connected to the low potential side of the capacitor 6 and having the other end connected to a connection point between resistors 11a and 11b connected in series between the power source and ground. The capacitor 10 and the resistors 11a and 11b form a high-pass filter.
The misfire detection circuit 5 also has a comparator 12 having a noninverting input terminal connected to the connection point between the high-pass filter capacitors 11a and 11b and having an inverting input terminal connected to a connection point between resistors 13a and 13b for setting a comparison reference voltage which are connected in series between the power source and ground. The comparator 12 detects the existence/non-existence of an ion current by comparing a voltage change caused by an ion current with the reference voltage. Further, one end of a resistor 14 is connected to the plus terminal of the primary coil 1a of the ignition coil 1, and a power stabilizing capacitor 15 and a voltage regulating diode 16 are connected between the other end of the resistor 14 and ground, thereby forming a power supply circuit of the misfire detection circuit 5.
In the thus-arranged circuit, when the internal combustion engine is ignited, the transistor 3 is abruptly changed from the ON state to the OFF state by the control of the controller for controlling combustion (not shown). At this time, the primary current of the ignition coil 1 decreases abruptly, so that a counter electromotive force is generated on the primary side to cause a voltage rise up to the collector-emitter withstand voltage of the transistor 3 (about 300 V). Simultaneously, on the secondary side of the ignition coil 1, the voltage generated on the primary side appears by being amplified by the ratio of the numbers of turns of the primary coil 1a and the secondary coil 1b. As a result, for example, a voltage of about -30 kV, is applied to the electrode of the ignition plug 2 to cause a spark.
In the circuit shown in FIG. 8, ignition energy is utilized to accumulate, in the capacitor 6, an amount of charge large enough to detect an ion current, and the voltage held by the capacitor 6 provides a high voltage of, for example, about 80 V set by the Zener diode 7 and applied to the ignition plug 2 immediately after ignition. A current thereby caused is detected as ion current. The current at the time of ignition flows in the direction opposite to the direction of arrow I5 in FIG. 8, and causes discharge at the ignition plug 2 to ignite and explode the air-fuel mixture in the combustion chamber 2A. This discharge current charges the capacitor 6 to the voltage limited by the Zener diode 7.
The ion current detecting operation of the misfire detection circuit 5 will be described with reference to the operation timing chart of FIG. 9, which represents a case where no leak current such as that mentioned later occurs.
The operation of the transistor 3 is controlled by the controller for controlling combustion (not shown). The transistor 3 is in the OFF state when the base voltage V.sub.3 is low level and in the ON state when the base voltage V.sub.3 is high level. When the base voltage V.sub.3 of the transistor 3 is changed from high level to low level, the potential V.sub.2 of the ignition plug 2 is reduced to, for example, about -30 kV by the counter electromotive force of the coil to cause a spark. As long as a voltage high enough to produce the spark is maintained, the ignition current flows in the direction opposite to the direction of arrow I5 in FIG. 8 to cause a voltage drop across the diode 8, so that the output after the bypass filter, i.e., the potential V.sub.12+ of the noninverting input terminal of the comparator 12, rises.
When the ignition circuit becomes unable to maintain the spark, the potential V.sub.2 of the ignition plug 2 rises abruptly to become equal to the voltage V.sub.6 (e.g., 80 V) held by the capacitor 6. At this time, by the application of the positive voltage V.sub.6 of the capacitor 6, an-ion current is caused to flow in the direction of arrow I5 shown in FIG. 8. The current in the direction of arrow I5 flows through the resistor 15 to cause a voltage drop. As a result, the potential V.sub.12+ of the noninverting input terminal of the comparator 12 becomes lower in proportion to the ion current. This ion current is generated immediately after ignition and ceases to flow in several milliseconds.
The above-described comparator 12 detects the existence/nonexistence of ion current by comparing a change in the potential V.sub.12+ of the noninverting input terminal due to an ion current with the potential V.sub.12- of the inverting input terminal set to the comparison reference voltage set value by the resistors 13a and 13b. In this example, when the potential V.sub.12+ of the noninverting input terminal of the comparator 12 becomes lower than the potential V.sub.12- of the inverting input terminal, the potential V.sub.12out of the output terminal becomes low level, thereby detecting ion current. When no ion current is detected, the potential V.sub.12out of the output terminal is high level.
The above-described apparatus for detecting misfire in the internal combustion engine entails a problem described below. If carbon or the like is attached to the ignition plug 2 in the combustion chamber 2A, the insulation resistance of the ignition plug 2 is reduced. The ignition plug 2 can spark strongly enough for the operation of the internal combustion engine if the insulation resistance is higher than about 1 M.OMEGA.. However, when a voltage is applied to the ignition plug 2 having a reduced insulation resistance, a certain leak current occurs which is determined by the applied voltage and the insulation resistance. At the time of ion current detection, such a leak current appears in a state of being superposed on an ion current.
That is, if the leak current thus generated is small, it is proportional to the voltage of the capacitor 6 since it is proportional to the applied voltage, and it is constant because the voltage of the capacitor 6 is constant. In this case, a voltage signal due to the leak current and having a small change with respect to time attenuates by the effect of the high-pass filter formed by the capacitor 10 and the resistors 11a and 11b, while only a signal due to the ion current and having a large change with respect to time passes the filter. As a result, the ion current can be detected normally. However, if the leak current is increased, the variation in the voltage of the capacitor 6 becomes so large that the leak current and the ion current cannot be discriminated from each other.
Such a bad influence of an increase in leak current will be explained with reference to FIG. 10 in comparison with FIG. 9.
The voltage V.sub.6 of the capacitor 6 has the value limited by the Zener diode 7 during the ignition period. When ignition is completed, discharge by the above-described leak current starts to reduce the voltage V.sub.6 with a time constant determined by the capacitance of the capacitor 6 and an insulation resistance of the ignition plug 2. Simultaneously, the potential V.sub.2 of the ignition plug 2 is also reduced because the voltage determined by the Zener diode 7 and held by the capacitor 6 (e.g., 80 V) cannot be maintained. Accordingly, by the influence of this leak current, a state continues where the potential V.sub.12+ of the noninverting input terminal of the comparator 12 is smaller than the potential V.sub.12- of the inverting input terminal set to the comparison reference voltage set value, even though an ion current is generated immediately after ignition and ceases to flow in several milliseconds. During this state, the potential V.sub.12out of the output terminal of the comparator is low level. As a result, a detection is erroneously made to determine the existence of an ion current, even if no ion current flows, thus reducing the ion current detection accuracy.